The bacterial reaction center (RC) is the membrane protein responsible for the initial light induced electron and proton transfer reactions responsible for conversion of light energy into chemical energy. A key component in this process is the bound quinone, Q/B which undergoes reaction, Q/B + 2e minus + 2 H+ equals Q/B H/2, coupling electron transfer with proton uptake from solution. This reaction is the first step in the proton pump that drives protons across the membrane using energy derived from electron transfer. The approach in this study is to use site directed mutagenesis to genetically modify key residues in the RC structure near the Q/B site and the to analyze the results of these changes by kinetic measurements, spectroscopic techniques and computational methods. The mutations are designed to identify the pathway of proton transfer and to test the role of electrostatic interactions and internal water molecules on proton transfer. Spectroscopic studies on native RCs and mutants will be conducted to identify key intermediates, such as the protonated semiquinone, in electron and proton transfer reactions. The ultimate goal of this project is to understand the structural basis for proton transfer into biological membranes, electron transfer between quinone molecules and the coupling between proton and electron transfer. These basic processes are responsible for energy conversion in many biological systems, such as mitochondria. The understanding of the structural basis for these processes can lead to elucidation of genetic defects that lead to metabolic deficiency and disease.